The present invention relates generally to polysiloxane resins. It relates specifically to ultraviolet radiation absorbing polysiloxane resins which may be applied to a transparent sheet using a roll coating process.
Polysiloxane resins are known to be useful as durable overcoating materials for plastics, especially for transparent plastics. A polysiloxane resin may be formed from an aqueous alcoholic composition including silanols. The resin so formed may be hardened by a process such as heat curing. Colloidal silica may be added to the silanol composition so that the polysiloxane resin formed from the composition includes silica. The silica containing resin may be harder than most transparent plastic materials and may be used as a hard overcoating for plastic materials. A hard overcoating may be used on plastic lenses for corrective or protective eyewear. A hard overcoating may also be used to prevent abrasion of plastic lenses for automotive headlamps.
A polysiloxane resin may include a material which absorbs ultraviolet light. Thus, a hard overcoating for a plastic eyeglass lens may provide ultraviolet protection for a user as well as abrasion resistance for the lens surface.
An ultraviolet-absorbing polysiloxane resin may also be useful as a coating for a protective glazing for a photograph or a work of graphic art. Preferably the protective glazing should absorb over ninety-nine percent of light wavelengths shorter than 400 nanometers (nm). The glazing may be a sheet of plastic or glass. Glass used for picture frame glazings does not usually absorb wavelengths greater than about 320 nm.
A plant for producing resin-coated picture frame glass may be required to produce about twenty-five thousand square feet of coated glass per day. The resin layer is preferably applied to the glass by a process known as roll coating. In roll coating, a flowable resin composition is applied continuously to an elongated rubber roller. A sheet of the glass to be coated is passed beneath the rubber roller to receive a layer of the resin composition. The layer of resin composition is then cured, generally by heat curing. The curing is preferably effected quickly, so that storage for large areas of uncured product is not required. Prolonged storage of an uncured layer may also increase the chance that dust or other foreign particles may be permanently included in the cured layer.
Polysiloxane resins useful for a hard overcoating for plastic are disclosed in U.S. Pat. Nos. 4,355,135; 4,390,660; 4,156,056; 4,396,228; 4,419,405; and 4,436,924. U.S. Pat. No. 4,355,155 discloses a base resin formulation which includes a catalyst and a curing or crosslinking agent to accelerate the resin-curing process at relatively low temperatures. The resin may be cured at a temperature as low as about 50.degree. C. It is also disclosed that the resin may be suitable for application by a number of methods including roll coating, although a roll coating process is not specifically described.
Problems have been encountered in applying a layer of ultraviolet-absorbing resin to a picture frame glazing using a roll coating process. Certain of these problems are discussed in detail in U.S. patent application Ser. No. 07/728,611, filed Jul. 11, 1991, by Stevenson et al., and entitled "A Protective Glazing and Process for making the same", and assigned to the assignee of the present application. For example, it was determined that the presence of a curing agent and catalyst in a resin composition is not in itself sufficient to promote rapid curing after a resin layer had been formed. It was found that a relatively high temperature of about 420.degree. F. may significantly reduce curing time. At high temperatures, however, ultraviolet-absorbing material included in the resin composition may be volatilized during the curing process, thus reducing the ultraviolet blocking effectiveness of the cured resin layer. Ultraviolet-absorbing material may also be removed from the resin layer if the coated glazing is subsequently cleaned by a wet process.
A curing agent and a catalyst in a resin composition may cause it to begin to cure before it is applied. Thus storage life of the resin composition may be reduced. Precipitates may also form in the resin composition if it includes a curing agent and a catalyst. The precipitates may be observed in the cured resin layer as a "sparkling" effect.
Excessively rapid uncontrolled curing of a resin layer may prevent it from uniformly covering a surface and/or forming a smooth surface.
Another problem, which may be encountered in resins of any kind including an ultraviolet-absorbing material, is color control. Specifically, if the ultraviolet-absorbing material fully absorbs wavelengths less than about four hundred nanometers, it may also at least partially absorb some wavelengths in the blue portion of the visible spectrum. If some portion of the blue region of the visible spectrum is absorbed in preference to the red and green regions, the resin layer may take on a yellow color. Excessive yellow color may be aesthetically unacceptable in a glazing for works of graphic art. Such a glazing preferably has a neutral transmission color.
Accordingly, it is an object of the present invention to provide an ultraviolet-absorbing base resin formulation which will retain its ultraviolet blocking properties during heat curing.
It is another object of the present invention to provide an ultraviolet-absorbing cured resin layer which will retain its ultraviolet-absorbing properties after it is cleaned by a wet process.
It is yet another object of the invention to provide an ultraviolet base resin formulation specifically for use in a roll-coating process.
It is a further object of the invention to provide an ultraviolet-absorbing resin layer which has a substantially neutral transmission color.